1. Field of the Invention
This invention relates to an improvement in measuring corrosion rates. More specifically, this invention provides an improved corrosion probe and a method for rapidly measuring corrosion rates of metals, or the like, in a corrosive liquid environment.
2. Description of the Prior Art
When crude oil is produced, it often comprises corrosive components, such as brines containing dissolved CO.sub.2 or H.sub.2 S. Carbon dioxide, naturally occurring or used as a "gas lift" to assist the production of crude oil, is one corrosive component that further increases the corrosivity of the crude oil. Corrosion destroys production tubing which can result in production losses and costly workover of the well.
Not all corrosive brine liquid environments, such as brine/crude oil/gas mixtures, induce the same rate of corrosion on metals. Some corrosive liquid environments corrode metals at a more rapid rate than others. It is desirable and cost effective to treat with a corrosive inhibitor those corrosive liquid environments, such as some brine/crude oil/gas mixtures, which are substantially corrosive while not treating or closely monitoring the treatment of those corrosive liquid environments which are negligibly corrosive. This is possible only if the combination of pressure, temperature, gas, and fluid compositions which will be corrosive can be predicted along with those metal alloys and inhibitor systems which will minimize corrosion. Reliable predictions and concomitant material selections are possible only if accelerated, accurate, and unequivocal corrosion rate measurements can first be made under controlled environmental and chemical conditions, which are identical to, or closely approximating those experienced in the field. If is therefore preferably, if not necessary, that corrosion rate measurements be made directly in corrosive liquid environments in order to obtain reliable predictions along with those metal alloys and inhibitors which will prevent corrosion or keep it at a minimum.
Electrochemical polarization measurements are widely used to measure the corrosion rates of metals in brines. The technique is rapid and accurate and, therefore, the preferred method for monitoring corrosivity. However, the electrical resistance of oil/brine mixtures can be many orders of magnitude greater than the brine itself. This results in an IR voltage correction (current x inter-electrode resistance) many times greater than the voltage involved in the corrosion process (e.g., 10 volts vs. 0.01 volts). This results in considerable scatter and error in the data, limiting the use of electrochemical polarization methods for measuring the corrosivity of oil/brine mixtures.
U.S. Pat. No. 3,331,021 to Marsh, et al. relates to a method and apparatus for measuring instantaneous corrosion rates of specimens exposed to corrosive electrolytes. U.S. Pat. No. 3,337,440 to Nestor teaches a novel and improved apparatus which may be employed to investigate the corrosive characteristics of metallic substances and/or the effectiveness of corrosion inhibitors in water insoluble non-conducting environments. U.S. Pat. No. 3,361,660 to Chittum, et al. provides an apparatus for measuring the electrochemical potential of the inner surface of a tank containing brine. The apparatus comprises a tubular member that can be inserted through an access opening, such as a gate valve that may be installed for such purpose, or one that is already available, in order to make electrolytic contact with the brine solution in the tank. U.S. Pat. No. 3,436,320 to Marsh discloses a method and apparatus for determining the redox current and the corrosion current in a redox solution. The electrodes include an electrode with a corrodible metal to be tested, and a reference electrode. The same direct current voltage is applied to an inert electrode and the reference electrode in measuring the currents. U.S. Pat. No. 3,406,101 to Kilpatrick is concerned with correlating the polarization characteristics of metals with the corrosion rate of the metals, and with the design of a compact and simple apparatus adapted to permit an easy and rapid determination of the rates at which the metals are corroding in electrolytic solutions by means of polarization measurements. U.S. Pat. No. 3,486,996 to Annand discloses a corrosion test probe assembly for determining the corrosion rate of metallic constructional material exposed to a corrodant electrolyte by means of polarization measurements. U.S. Pat. No. 3,518,530 by Wilson teaches the procurement and use of polarization characteristics of metals such that when placed in underground formations, either directly or in cooperation with other electrical properties of the formations, determine the nature of the formation and/or the fluid contents of those formations. U.S. Pat. No. 3,607,673 to Seyl discloses a system for correcting for the IR drop between electrodes when corrosion rate of the electrodes is measured by applying a small dc voltage across the electrodes. U.S. Pat. No. 4,040,931 to Wilson relates to the instruments and the electrochemical techniques used in measuring and testing corrosion processes.
None of the foregoing prior art teaches or suggests the particular corrosion probe, the apparatus, or the processes of this invention for measuring the corrosion rates of metals in a corrosive liquid environment.